1. Field of the Invention
The present invention relates to a communication apparatus based on an OFDM (Orthogonal Frequency Division Multiplexing) system.
2. Description of the Related Art
A conventional OFDM-based communication apparatus generally uses coherent detection or delay detection as a demodulation system. First, a conventional OFDM communication apparatus that carries out coherent detection will be explained with reference to FIG. 1. FIG. 1 is a block diagram showing a configuration of a conventional OFDM communication apparatus that carries out coherent detection.
In the transmission system in FIG. 1, a transmission signal is subjected to QPSK modulation processing for every subcarrier by QPSK modulation section 41. The transmission signal subjected to QPSK modulation is subjected to IFFT (inverse Fourier transform) processing and thereby frequency-division multiplexed by IFFT section 42. The transmission signal subjected to IFFT processing is transmitted via antenna 43.
The signal transmitted via antenna 43 has a frame configuration as shown in FIG. 2. FIG. 2 is a schematic diagram showing a frame format used for the conventional OFDM communication apparatus. As shown in FIG. 2, the signal transmitted via antenna 43 is configured by preamble section 51, pilot symbol section 52 and message section 53 in an old-to-new time sequence. Each signal with this frame configuration is sent carried on one packet.
In the reception system in FIG. 1, the signal sent from the other end of communication is received via antenna 43. Here, the other end of communication above has the same configuration as that shown in FIG. 1 and the signal sent from this other end of communication is subjected to the same processing as that in the transmission system described above.
The signal received via antenna 43 is subjected to FFT (Fourier transform) processing by FFT section 44. Thus, a signal carried by each subcarrier is extracted. The signal extracted by FFT section 44 is subjected to coherent detection processing by coherent detection section 45. In this way, a demodulated signal is extracted.
Here, the coherent detection processing carried out by coherent detection section 45 will be explained with reference to FIG. 3. FIG. 3 is a block diagram showing an internal configuration of coherent detection section 45 in the conventional OFDM transmission apparatus that carries out coherent detection.
Coherent detection section 45 performs transmission path estimation using a pilot symbol in the reception signal and performs coherent detection processing by carrying out transmission path compensation on the reception signal using the transmission path estimation information obtained.
In FIG. 3, the reception signal (RX1) is sent to complex multiplication section 62, level detection section 63 and multiplication section 65 via switching section 61. This reception signal (RX1) is the signal extracted from FFT section 44 shown in FIG. 1.
Here, the reception signal (RX1) is expressed in the following expression:RX1=R1×ejθ1×TX  {circle around (1)}                where, R1 is an amplitude variation due to fading, etc. and θ 1 is a phase variation due to fading, etc. and TX is a signal transmitted from the other end of communication (transmission signal).        
Moreover, the signal in the pilot section of the reception signal expressed in expression {circle around (1)} above, that is, the reception signal (RXP1) in the pilot section is expressed in the following expression:RXP1=R1×ejθ1×Pilot  {circle around (2)}                where, Pilot is a pilot symbol.        
Complex multiplication section 62 estimates a transmission path characteristic by carrying out complex multiplication processing using the reception signal (RX1) and pilot symbol (Pilot). That is, a transmission path characteristic (Profile1) as shown in the following expression is obtained by multiplying the reception signal (RXP1) in the pilot symbol section expressed in expression {circle around (2)} above by a conjugate complex number (Pilot*) of the pilot symbol (Pilot).Profile1=R1×ejθ1×Pilot×Pilot*=R1×ejθ1×|Pilot|2  {circle around (3)}                where, if |Pilot|2=1, expression {circle around (3)} above can be expressed in the following expression:Profile1=R1×ejθ1  {circle around (4)}        
The transmission path characteristic (Profile1) obtained is sent to division section 64.
On the other hand, level detection section 63 calculates reception power of the reception signal (RX1). The reception power of the reception signal (RX1) is R12 from expression {circle around (1)} above. The reception power of the reception signal (RX1) is sent to division section 64.
Division section 64 performs the following division processing using the transmission path characteristic (Profile1) from complex multiplication section 62 and the reception power from level detection section 63.Pfifile1/R12=ejθ1/R1  {circle around (5)}
The result of the division processing in division section 64 is sent to multiplication section 65.
Multiplication section 65 performs transmission path compensation on the reception signal using the result of the division processing in division section 64. That is, a demodulated signal is obtained by multiplying the reception signal (RX1) from switching section 61 by the conjugate complex number in expression {circle around (5)} above as shown in the following expression:RX1×ej−θ1/R1=R1×ejθ1×TX×ej−θ1/R1=TX  {circle around (6)}
Then, a conventional OFDM communication-apparatus that carries out delay detection will be explained with reference to FIG. 4. FIG. 4 is a block diagram showing a configuration of an OFDM communication apparatus that carries out delay detection. The OFDM communication apparatus shown in FIG. 4 has a configuration including DQPSK modulation section 71 and delay detection section 72 instead of QPSK modulation section 41 and coherent detection section 45 in FIG. 1. As a frame format, the one shown in FIG. 2 can be used.
DQPSK modulation section 71 performs differential coding QPSK modulation (generally referred to as “DQPSK modulation”) on the transmission signal. Delay detection section 72 performs delay detection processing on the reception signal (RX1) by multiplying the signal at the current time by the signal preceding by 1 OFDM symbol. Through this multiplication processing, a demodulated signal is obtained.
However, the conventional OFDM communication apparatus above has problems as shown below. That is, in the conventional OFDM communication apparatus that carries out coherent detection, when the communication speed of one packet is slower than the channel variation speed, the channel condition at the time of reception of the pilot symbol section in each packet differs from the channel condition at the time of reception of the message section of the packet above. Thus, transmission path compensation is performed on the message section above using the transmission path characteristic estimated by the pilot symbol section above, and therefore the error rate characteristic of the demodulated signal obtained in the message section above deteriorates a great deal.
On the other hand, the conventional OFDM communication apparatus that carries out delay detection outputs the result of multiplying the signal at the current time by the signal preceding by 1 OFDM symbol as a demodulated signal, and therefore an error caused by a transmission path variation included in this demodulated signal is only 1 OFDM symbol. Thus, if the communication speed of one packet is slower than the channel variation speed, deterioration of the error rate characteristic of the demodulated signal is small.
On the other hand, in the conventional OFDM communication apparatus that carries out delay detection, the noise component superimposed on the demodulated signal doubles by multiplying the signal at the current time by the signal preceding by 1 OFDM symbol, and therefore if the communication speed of one packet is sufficiently faster than the channel variation speed, the error rate characteristic of the demodulated signal deteriorates compared to the conventional OFDM communication apparatus that carries out coherent detection.
As shown above, in the conventional OFDM communication apparatuses above, the error rate characteristic of the demodulated signal may deteriorate depending on the relationship between the communication speed of one packet and channel variation speed.